G-protein-coupled receptors (GPCRs), also known as 7-transmembrane receptors, are the single largest class of drug targets, with more than 800 members in the human genome (Lefkowitz, Trends in Pharmacological Sciences (2004), 413). Dopamine receptors represent prototypic examples of GPCRs that mediate neurotransmission (Missale et al., Physiological Reviews (1998), 189). Dopamine is a monoamine neurotransmitter that exerts its action on neuronal circuitry via dopamine receptors. As dopaminergic innervations are most prominent in the brain, dopaminergic dysfunction can critically affect vital central nervous system (CNS) functions, ranging from voluntary movement, feeding, reward, affection, sleep, attention, working memory and learning (Carlsson, Science (2001), 1021, Beaulieu et al., Pharmacological Reviews (2011), 182). Apart from CNS functions, dopamine is also involved in important physiological roles such as the regulation of olfaction, cardiovascular functions, sympathetic regulation, hormonal regulation, retinal processes, immune system and renal function. Dysregulation of dopaminergic neurotransmission has been associated with multiple neurological and psychiatric conditions such as Parkinson's disease, Huntington's disease, attention deficit hyperactivity disorder (ADHD), mood disorders and schizophrenia (Carlsson, Science (2001), 1021), as well as various somatic disorders such as hypertension and kidney dysfunction (Missale et al., Physiological Reviews (1998), 189, Beaulieu et al., Pharmacological Reviews (2011), 182).
With the complex array of critical cellular functions mediated by dopamine receptors, and the multilevel interactions that are known to occur between dopamine and other extracellular messengers in the signaling pathways, there remains a need to better manage dopamine-related pathologic conditions by precise targeting of post-receptor intracellular signaling modalities, either directly or through ligand-biased signaling pharmacology.
As drug targets, GPCRs known to mediate dopamine functions can be broadly classified into D1 and D2 class receptors. D1 class receptors (D1R and D5R) are mostly coupled to Gas and positively regulate the production of second messenger cAMP and the activity of protein kinase A (PKA) (Missale et al., Physiological Reviews (1998), 189). D2 class receptors (D2R, D3R and D4R) couple to Gαi/o, downregulating cAMP production and PKA activity (Missale et al., Physiological Reviews (1998), 189). Additionally, D2 class dopamine receptors also modulate intracellular Ca2+ levels, resulting in changes in activity of Ca2+ regulated signaling proteins such as protein phosphatase calcineurin (Nishi et al., J. Neurosci., 1997, 17, 8147).
D2 class dopamine (D2R) receptors are presently the best-established targets for antipsychotic drugs. Recent studies suggest that β-arrestin 2 deficiency in mice results in reduction of dopamine-dependent behaviours (Beaulieu et al., Cell (2005), 261). The connection between β-arrestin 2 and dopamine-associated behaviours suggests that β-arrestin 2 could be a positive mediator of dopaminergic synaptic transmission and a potential pharmacological target for dopamine-related psychiatric disorders (Beaulieu et al., Cell (2005), 261).
Currently, all clinically marketed antipsychotics modulate dopamine by targeting D2R either as antagonists/inverse agonists (first- and second-generation antipsychotics, for example, chlorpromazine, clozapine) or partial agonists (third-generation antipsychotics, with aripiprazole as the sole example of this ligand class in the clinic). Antagonism of dopamine D2 receptor/β-arrestin 2 interaction has been found to be a common property of clinically-effective antipsychotics (Masri et al., Proceedings of the National Academy of Sciences of the United States of America (2008), 13656).
Structure-functional selectivity relationship studies of β-arrestin-biased dopamine D2 receptor agonists, based on the aripiprazole scaffold, have been conducted (Chen et al., Journal of Medicinal Chemistry (2012), 7141, Roth et al., US 2013/0137679, Shonberg et al., Journal of Medicinal Chemistry (2013), 9199). Known antipsychotics, even those that share a common mechanistic pathway such as haloperidol, clozapine, and risperidone, show highly diverse effects on D2R/G protein signaling and are not selective across GPCR receptors. There remains a lack of clinical drug candidates that offer highly functionalized targeting of dopamine D2 receptors that improve the clinical efficacy of antipsychotics, while at the same time limiting the undesirable side effects associated with D2-dopaminergic activity.
Selectively antagonizing the β-arrestin pathway at the D2 receptor could be sufficient to produce an antipsychotic effect, while at the same time, reduce potential side effects that could arise from antagonizing the cAMP pathway. Modulation of the β-arrestin-2 dependent pathway could lead to modulation of AKT and GSK3β target genes (Beaulieu et al., Frontiers in molecular neuroscience (2011), 38.). Development of compounds with cAMP biased agonist or antagonist or β-arrestin biased agonist or antagonist activity could offer a functionally selective means to modulate or treat dopamine-associated disorders, including Parkinson's disease, Huntington's disease, mood disorders, schizophrenia, attention deficit hyperactivity disorder (ADHD), restless legs syndrome (RLS), pituitary disorders such as pituitary adenoma or pituitary tumor (prolactinoma) or endocrine disorders, e.g., galactorrhea. Further, development of ligands that exhibit functional selectivity as agonists, antagonists, and partial agonists, as well as selectivity against other GPCRs, allows modulation of activity at the dopamine D2 receptors to be more finely-tuned to increase selectivity and hence clinical efficacy and safety in treatment. By increasing selectivity at dopamine D2 receptors while minimising undesirable side-effects, drugs in this category would also offer greater success potential with patient acceptance and compliance.